Optical transmission systems are generally based on one of two methods of modulation of a signal laser, either direct or external modulation. In the first of these, the bias current to the laser is modulated, turning the laser on and off. The disadvantage of this when applied to high capacity systems is that the semiconductor material dynamic behaviour introduces distortion into the laser output, known as chirp. External modulation of the continuous wave (CW) source produces a modulated output signal with significantly reduced chirp, and sources of this type are preferred for use in high capacity systems. High speed electrode-optic modulators such as Mach Zehnder devices are typically used.
One coding format for optical signals in fibre optic networks is non-return-to-zero (NRZ). In this format a coded data pulse fills the time slot corresponding to a bit period and if the next data bit has the same value no transition occurs at the end of the time slot. An example of a modulator which is designed to implement NRZ format transmission is the X2624C Mach Zehnder opto-electronic device available from Lucent. This is a dual electrode Mach Zehnder device which uses an electrical data stream and its inverse to drive respective electrodes, thereby gating (modulating) a CW source to provide NRZ optical data. By altering the two drive levels on the respective electrodes it is possible to affect a degree of pre-chirp (positive or negative) to compensate for non-linear transmission effects.
An alternative to NRZ coding is to use a return-to-zero (RZ) format. In this case, if the data in a time slot is a xe2x80x9c1xe2x80x9d, the signal drops back to the xe2x80x9c0xe2x80x9d level before the end of the slot, whatever the value of the data in the next bit: a xe2x80x9c1xe2x80x9d is signified by the presence of a pulse located within the bit period. RZ format is more stable than NRZ coded optical signals at high bit rates, and is therefore preferred for high capacity long haul transmission systems.
One known arrangement for coding an RZ optical signal requires a first Mach Zehnder device to modulate a CW source using NRZ electrical data as the drive signal, the output of which is modulated by a second Mach Zehnder device configured as an optical AND gate to convert the NRZ optical input to an RZ optical signal at the output. The output of the second Mach Zehnder device is then coupled to a phase modulator to provide a degree of pre-chirp to compensate for non-linear transmission effects. It is not possible to provide an integrated package and therefore this solution is much more complex.
The latest 10 Gbit sxe2x88x921 WDM optical transmission systems that are being proposed for submarine networks will provide 60 or more channels with a spacing of only 0.4 nm. In future, 120 channels having a spacing of only 0.2 nm will be offered. One problem with the development of high capacity systems such as these is that the footprint of the terminals required to support the systems are becoming increasingly large at a time when customers are demanding ever smaller integrated solutions to their needs. The preferred RZ coding format for high bit rate systems and the associated optical devices described above required to generate an RZ coded signal from an NRZ electrical data stream makes this difficult to achieve.
According to a first aspect of the present invention, an optical device for the external modulation of the output of an optical source, comprises:
a first optical modulation device driven by an NRZ coded electrical data stream to modulate the output of the optical source to generate an NRZ coded optical signal; and,
a second optical modulation device coupled to the output of the first optical modulation device and driven by one or more sources to implement an optical AND function and controllable to introduce a predetermined degree of phase modulation to thereby convert the NRZ coded optical signal from the first optical modulation device to a phase modulated RZ coded optical signal for subsequent transmission.
According to a second aspect of the present invention, an optical modulator comprises a dual electrode Mach Zehnder device driven by one or more sources to implement an optical AND function and thereby convert an NRZ coded optical signal at an input to an RZ coded optical signal at an output.
Preferably, the one or more sources comprise an electrical circuit for controlling the drive of electrodes of the dual electrode Mach Zehnder device to modulate the phase of the coded optical signal and thereby introduce a predetermined degree of pre-chirp to the RZ coded optical signal at the output.